The present invention relates to the field of combustors in gas turbines and specifically to pilot air technology for such combustors.
Gas turbines mix and combust fuel and compressed air in a combustor arranged between a compressor and turbine. Combustors for industrial gas turbines typically include an annular array of combustion cans that each include fuel and air nozzles. The combustion cans, fuel and air nozzles and other components of the combustors are arranged to provide efficient and low emission combustion of high pressure and high mass flow rates of compressed air, and liquid and/or gaseous fuel. The combustion system often includes primary and secondary fuel nozzles for liquid and gaseous fuels, and associated piping for the different fuel types. Water injection pipes and nozzles are also included in some combustion systems.
Pilot air has been applied to gas turbine combustors to (for example): assist in gaseous fuel combustion; purge secondary fuel pipes and nozzles, and purge water injection pipes and nozzles in the combustion system. Pilot air has also been used in conjunction with emission control technology that reduces nitrogen oxides (NOx) emissions from the combustion process. Compressed air taken from the main compressor is a common source of pilot air. The pilot air bled off from the main compressor may be boosted by a secondary compressor and applied to the combustor. Conventionally, the boosted pressure of pilot air has not been regulated or adjustable. There is a need to regulate the pressure level of the pilot air, especially in view of the different applications of the pilot air, e.g., for purging fuel and water injection pipes, assisting gaseous fuel flow, and for emission control.